The NOAA Climate Prediction Center (CPC) noted earlier this month that “La Niña conditions are expected to strengthen and last through the Northern Hemisphere winter 2010-11.” A La Niña occurs when abnormally cold waters develop along the equator of the central and eastern Pacific Ocean. These changes in the ocean and atmosphere in turn influence the weather over the United States.
That feature is incorporated into the seasonal outlooks provided by the CPC. Right now they are calling for an increased chance of above normal temperatures this fall (September-November) across Illinois. Precipitation has an increased chance of being below normal in the southern two-thirds of Illinois.
For this winter (December-February), they call for an increased chance of above normal temperatures in the southern half of Illinois, with equal chances of above, below and near normal temperatures in the northern half of Illinois. They do call for an increased chance of above normal precipitation across all of Illinois and much of the Ohio River Valley. Updated official seasonal forecasts are posted mid-month here.
As of August 16, cooling degree days (CDD) are well above normal this year in Illinois. The statewide average is 1051 CDDs, which is 268 CDDs above normal or 134 percent of normal. Last year the statewide average through this date was only 640 CDDs.
Cooling Degree Day accumulations at selected cities.
Departure from Normal
Percent of Normal
Cooling degree days are calculated by subtracting the mean daily temperature by 65 degrees. Results above zero are accumulated over time. So if the mean temperature for the day was 75 degrees, then 75 – 65 = 10 cooling degree days. Accumulated over a season and compared to normal gives a relative idea of potential cooling costs. The higher the number, the higher the cooling costs.
Other factors that can influence cooling costs are humidity levels and solar radiation. Personally, I ran the A/C at home pretty much every day since mid-May. Last year I ran it for about two weeks the entire summer.
Record of Days At or Above 80 Degrees for Chicagoland
THE PREVIOUS RECORD FOR CONSECUTIVE 80 DEGREE OR ABOVE DAYS WAS 42
DAYS IN A ROW...SET IN 1955. ON AUGUST 13 2010...THE CHICAGOLAND
AREA HIT 43 CONSECUTIVE DAYS...AND THE NUMBER CONTINUES TO
INCREASE. JULY 2 2010 WAS THE FIRST 80 DEGREE DAY OF THE CURRENT
RECORD. AS OF 251 PM CDT TODAY...WHEN THE TEMPERATURE AT OHARE
INTERNATIONAL AIRPORT OFFICIALLY REACHED 80 DEGREES DURING
AFTERNOON HEATING...TODAY IS THE 46 CONSECUTIVE DAY FOR THIS
THE TOP TEN RECORDS ARE AS FOLLOWS
1. 46 DAYS...WITH THE STREAK STILL CONTINUING ON AUGUST 16 2010
2. 42 DAYS...WITH THE STREAK ENDING ON AUGUST 7 1955
3. 34 DAYS...WITH THE STREAK ENDING ON SEPTEMBER 6 1995
4. 32 DAYS...WITH THE STREAK ENDING ON AUGUST 8 1983
5. 30 DAYS...WITH THE STREAK ENDING ON JULY 19 1921
6. 29 DAYS...WITH THE STREAK ENDING ON AUGUST 18 1988
7. 27 DAYS...WITH THE STREAK ENDING ON AUGUST 6 1999
8. 27 DAYS...WITH THE STREAK ENDING ON JULY 14 1966
9. 25 DAYS...WITH THE STREAK ENDING ON AUGUST 15 2007
10. 24 DAYS...WITH THE STREAK ENDING ON JULY 26 2005
The state-wide temperature in Illinois for the first half of August (August 1-15) was 79.5 degrees, 5.7 degrees above normal. That should come as no surprise to anyone who experience the hot, humid conditions in August.
The state-wide precipitation for the first half of August was 2.32 inches, just 0.41 inches above normal. The wettest conditions were in the northern third of Illinois with most locations reporting 2 to 5 inches of rain. Meanwhile parts of eastern and southeastern Illinois have been on the dry side, reporting less than 1.0 inches so far in August. The rest of the state was close to normal (around 2 inches).
While this summer has been hot and humid, it pales in comparison to the hottest month in Illinois history – July 1936. Here are the records from July of that year at Midway Airport in Chicago. The temperature hit the triple digit mark nine times that month. That’s more times than for the entire 1954 to present-day period at Midway Airport. Also, eight of those nine days in July were consecutive.
It was also very dry with only 0.31 inches of precipitation that month. A “T” in the precipitation column means the a trace event – enough to wet the sidewalk but not enough to measure.
The second table has the reports from the University of Chicago site. Those numbers show that conditions closer to the lake were a little more tolerable that summer but still very warm. And this was all before air conditioning was in widespread use.
While much attention has been given to the areas of heavy rain in Illinois, there were two areas of significant dryness in the state in July. A product from the NWS that combines radar estimates and rain gauge measurements better defined those areas. See figure below.
One area of dryness in July was across north-central Illinois. Amounts in this area were only 25-75 percent of normal. Fortunately, this area received ample rain in June. Even so, the drying of the topsoil is clear in the August 2 USDA NASS Illinois Weather and Crops report.
The other area of dryness in July was in southern Illinois, south of I-64. This area was dry in June as well. As a result of two consecutive months of dry weather, the U.S. Drought Monitor designated this area as “abnormally dry.”
Here is the link to the NWS product: http://water.weather.gov/precip/
Here are the thumbnail maps (click to enlarge) of July temperature departures, and July rainfall and rainfall departures. It was warmer than normal across much of eastern Corn Belt. It was much wetter than normal across much of the western Corn Belt.
The average temperature for July in Illinois was 77.7 degrees, 1.9 degrees above normal. That’s warm but not record-breaking by any means. By contrast, the average temperature of last July was 70.2 degrees. Not only was this year warmer but it was considerably more humid as well.
The average rainfall for July in Illinois was 5.6 inches, 1.8 inches above normal. BTW, the wettest July on record was 1958 with 8.03 inches.
The largest rainfall totals occurred in western and far northern Illinois as well as an area along I-70. Amounts of 8 to 12 inches were common in these areas.
Southern Illinois was much drier in July with amounts of only 1 to 3 inches in many locations. In fact, the US Drought Monitor categorized southern Illinois as being “abnormally dry” by. Another area of dryness is through central Illinois in an area bound by Moline, Kankakee, Danville, Springfield, and Peoria.
Here are some rainfall totals for July 2010 [updated on August 3] for the Chicago area. This is preliminary and may contain missing or incorrect data. Missing data can make the monthly totals too low. Based on what has been reported so far, Midway Airport has set a new record for July precipitation, beating the old record of 8.98 inches set in 1957.
Monthly Data for July 2010 for Illinois – Northeast Climate Division
The statewide average temperature for July 1-22, 2010, in Illinois is 77.2 degrees, 1.1 degrees above normal for this time period. The state average precipitation for the same period is 3.46 inches, 0.70 inches above normal.
Northern and eastern Illinois are a little drier on average. However, even those drier areas contain locations with very impressive rainfall amounts.
Table 1. Temperature and precipitation amounts and departures from normal by climate division in Illinois for July 1-22, 2010. A map of the climate divisions can be seen below the table.